Saudi Arabia launched airstrikes in Yemen early Thursday in an attempt to roll back the advances of the northern rebel Houthi movement and in support of the president it helped install in 2011.

On Wednesday Houthi militia fighters were reported to be advancing on the southern port city of Aden. Yemeni President Abdu Mansur Hadi, who earlier had been driven out of the capital, Sanaa, by the Houthis, appears to have fled Aden sometime during the day.

Saudi Arabia announced the airstrikes at a press conference late Wednesday in Washington. They appear to have been focused on targets in and around Sanaa. The extent and impact of those remain unclear.

The Saudis said they’re acting as part of a coalition of members of the Gulf Cooperation Council (GCC), which combines the oil-rich Sunni Arab monarchies. Their capacity for sustained war even in a close neighbor like Yemen remains to be seen; Gulf militaries are generally built with attention to controlling the monarchs’ subjects rather than fighting wars abroad.

Four Egyptian warships entered the Suez Canal Thursday en route to the Gulf of Aden, and expressions of support for the Saudi operation came from around the region.

The US is backing the Saudis’ rare foray in direct power projection, according to National Security Council spokeswoman Bernadette Meehan.

The United States coordinates closely with Saudi Arabia and our GCC partners on issues related to their security and our shared interests. In support of GCC actions to defend against Houthi violence, President Obama has authorized the provision of logistical and intelligence support to GCC-led military operations. While US forces are not taking direct military action in Yemen in support of this effort, we are establishing a Joint Planning Cell with Saudi Arabia to coordinate US military and intelligence support. …

We strongly urge the Houthis to halt immediately their destabilizing military actions and return to negotiations as part of the political dialogue. The international community has spoken clearly through the UN Security Council and in other fora that the violent takeover of Yemen by an armed faction is unacceptable and that a legitimate political transition – long sought by the Yemeni people – can be accomplished only through political negotiations and a consensus agreement among all of the parties.

Yemen was divided into two countries for almost 25 years – a result of a civil war that began in 1962, in which Saudi Arabia was deeply involved. Yemen’s big northern neighbor was terrified at the thought of republican government emerging on its southern border.

North Yemen and South Yemen were reunified in 1990, but the country’s regional rivalries persisted and are driving the current conflict. The Zaydi Shiite Houthis have received backing from Iran, but have also aligned with former Yemeni President Ali Abdullah Saleh, who was driven from power in 2012 following bloody protests inspired by the uprisings in Egypt and Tunisia.

Mr. Saleh had ruled as a dictator for 30 years, with staunch backing from both Saudi Arabia and the US. During a visit to Washington in 2007, Saleh was warmly greeted by President George W. Bush as a friend and ally. Warm relations persisted into the Obama administration, with the US focused on securing Saleh’s support for carrying out its drone assassination campaign against Al Qaeda in the Arabian Peninsula. Saleh was happy to comply in exchange for money and weapons.

But since being nudged from power – the Saudis helped get him a deal that provided him immunity from prosecution for killing civilians – Saleh has made new friends in the Houthis, which he and the Yemeni military units that have remained loyal to him see as a path back to power.

As they edge closer to Aden, it has become clear the bulk of the forces heading south are military, rather than the Huthi rebels who hail from the northern mountains, according to local officials. And at times it has been apparent that some officers have acted under Saleh’s influence, allowing the rebels to capture territory or military positions without a single shot being fired.

“There are certainly orders in the chain of command that can be traced back to Saleh,” said Laurent Bonnefoy, a specialist on Yemen.

Although he is a member of the Zaidi sect of Shiite Islam to which the Huthis belong, Saleh waged six wars against the Shiite movement in northern Yemen between 2004 and 2010. Despite this, “there are numerous indications that Ali Abdullah Saleh has aligned himself with the Huthis to destroy the power base and property of his enemies,” said the expert report submitted to the UN.

As the US supported Saudi airstrikes against the Iranian-backed Houthis, it also conducted airstrikes of its own directly in support of an Iranian-backed militia in Iraq. US warplanes came to the assistance of the bogged-down government offensive on Tikrit yesterday, where the bulk of the fighters are Shiite militias trained and organized by Iran.

The Saudis, which share the Arabian peninsula with Yemen, have long treated the country as a potential threat, particularly because of the absence of monarchical control there and presence of raucous party politics.

Brian Whitaker writes that the longstanding Saudi policy designed to keep Yemen weak and divided doesn’t bode well for any military engagement. Saudi attempts at keeping Yemen “wobbly” have increased the country’s poverty and in many cases directly fed into its conflicts, as now.

The Houthi rebellion, though, was partly a result of Saudi missionary activities. A major factor leading up to the Houthi conflict was rivalry between the majority of Zaidi Shiites and a growing minority of men who had converted from Zaidism to the salafi or Wahhabi version of Sunni Islam, according to Shelagh Weir, a veteran Yemen watcher.

Though ostensibly religious, this rivalry also had a social dimension, Weir told a conference in London. Converts included men who occupied the bottom of the traditional status hierarchy and bitterly resented their social disadvantage, as well as youths who resented the power of the older generation or were attracted by the charisma of salafi leaders and their obvious financial resources. “Certain sheikhs openly or tacitly supported salafism for personal or anti-Zaidi reasons or because of the subsidies they received from Saudi Arabia.” …

Inevitably, the aggressive salafi/Wahhabi proselytising triggered a response from the other side, with the Houthis seeking to defend Zaidi rights in the Saadah region.

Saudi Arabia’s successful campaign to eradicate al-Qaeda from the kingdom also had the effect of driving militants into Yemen and caused AQAP, the local branch of al-Qaeda, to focus its attention there. Since the Houthis and al-Qaeda are sworn enemies, that also exacerbated the problems in Yemen.